Monday, March 12, 2012

RESULT_CACHE parameter 11g to improve query performance

Oracle 11g allows the results of SQL queries to be cached in the SGA and reused to improve performance. Set up the following schema objects to see how the SQL query cache works.

CREATE TABLE test_tab (

id NUMBER

);

INSERT INTO test_tab VALUES (1);

INSERT INTO test_tab VALUES (2);

INSERT INTO test_tab VALUES (3);

INSERT INTO test_tab VALUES (4);

INSERT INTO test_tab VALUES (5);

CREATE OR REPLACE FUNCTION slow_function(p_id IN test_tab.id%TYPE)

RETURN test_tab.id%TYPE DETERMINISTIC AS

BEGIN

DBMS_LOCK.sleep(1);

RETURN p_id;

END;

/

SET TIMING ON

The function contains a one second sleep so we can easily detect if it has been executed by checking the elapsed time of the query.

Next, we query the test table using the slow function and check out the elapsed time. Each run takes approximately five seconds, one second sleep for each row queried.

SELECT slow_function(id) FROM test_tab;

SLOW_FUNCTION(ID)

-----------------

1

2

3

4

5

5 rows selected.

Elapsed: 00:00:05.15

Adding the RESULT_CACHE hint to the query tells the server to attempt to retrieve the information from the result cache. If the information is not present, it will cache the results of the query provided there is enough room in the result cache. Since we have no cached results, we would expect the first run to take approximately five seconds, but subsequent runs to be much quicker.

SELECT /*+ result_cache */ slow_function(id) FROM test_tab;

SLOW_FUNCTION(ID)

-----------------

1

2

3

4

5

5 rows selected.

Elapsed: 00:00:05.20

SELECT /*+ result_cache */ slow_function(id) FROM test_tab;

SLOW_FUNCTION(ID)

-----------------

1

2

3

4

5

5 rows selected.

Elapsed: 00:00:00.15

The default action of the result cache is controlled by the RESULT_CACHE_MODE parameter. When it is set to MANUAL, the RESULT_CACHE hint must be used for a query to access the result cache.